1. Field of the Invention
The present invention relates to a mold for forming an optical element and a method of pressmolding, and more particularly to a mold for molding an optical element which enables the continuous manufacture of the optical element having a bevelled portion at a low cost and a method of press-molding using the same. The present invention further relates to a lens produced by such a molding method.
2. Related Background Art
A method for manufacturing an optical element having a high precision optical function plane, which does not need post-machining such as grinding and polishing by accommodating a material to be molded into the optical element such as a glass blank premolded to have general shape and surface precision in a mold having a predetermined surface precision and press-molding the material under heat, has been developed.
In such a press-molding method, an upper molding member and a lower molding member are slidably arranged in a molding body to face each other, a molding material is introduced into a cavity formed by the upper molding member, the lower molding member and the molding body. The molding members are heated to a molding temperature such as a temperature to render the material to 10.sup.8 to 10.sup.12 poise in a non-oxidization in order to prevent the molding members from being oxidized. Then, the mold is closed and the material is pressed for a proper time period to transfer the shape of the surfaces of the molding members to the surface of the material. After the temperature of the molding members is decreased to a sufficiently lower temperature than a glass transition temperature of the material, the mold is open to take out the molded optical element.
The optical element thus formed, for example, a lens abuts against a reference plane of a body tube when it is assembled into a lens-barrel. A bevelled portion may be formed on at least one outer periphery of the lens surfaces as a reference plane for the abutment. In a lens having a planar reference plane on an outer periphery of an optical function plane, bevelled portions are formed on the outermost peripheries of the reference planes on the both surfaces. The bevelled portion may also be formed to provide a clearance in the assembling or to prevent a failure of the lens.
The bevelled portion may be formed by grinding after the press-molding, but it is desirable to form the bevelled portion simultaneously upon the press-molding from standpoints of speed-up of manufacturing of the optical element and cost reduction.
In this respect, a method has been proposed, in which a molding ring member is mounted on the outer periphery of a lower molding member or an upper molding member, and the molding ring member is constructed so as to have a shape for forming the bevel at an end thereof, whereby the bevelled portion is formed in the optical element at the same time upon the press-molding (refer to Japanese Patent Application Laid-open Nos. 62-297233 and 1-167242).
However, in the above method in which the molding ring member is attached, an accuracy error due to the engagement between the ring member and the upper or lower molding member is involved. Particularly, it is difficult to attain a vertical alignment and there is the possibility of forming a flash in the press-molding. In order to avoid the above problem, it is necessary to enhance the manufacturing precision of the ring member and the upper and lower molding member. This increases the cost. Further, as the press-molding is repeatedly carried out, the engagement between the upper or lower molding member and the ring member becomes loose due to a thermal hysteresis and a mechanical stress, thereby to lower gradually the shape precision of the molded product.